The steady tide of black migration out of Virginia, which started after the Civil War and continued for about a century has now stopped, according to a new U.S. Census Bureau report, which says that more blacks are moving into the state in the 1970s than are leaving it.

The net inflow of blacks still is not large - about 28,800 from 1970 to 1975, or 3.3 percent.

But it is in sharp contrast to what happened earlier. During the 1960s, for example, 79,000 more blacks left Virginia than moved into the state.

According to the new report, four other southern states - Florida, Georgia, South Carolina and Texas - also reversed historic patterns and attracted more blacks than they lost during the first half of the 1970s.

On the other hand, the District of Columbia has its first net outmigration of blacks ever recorded in census reports - 53,300 from 1970 to 1975, the Census Bureau said. Eight northern and midwestern states whose industries attracted blacks from the South for decades, also had a net black outflow from 1970.

In Maryland, the Census Bureau reported, the rate of net black inmigration since 1970 is almost double what it had been during the 1960s. The actual difference in the number of black newcomers to Maryland over those who left - 87,400 from 1970 to 1975 - was greater than in any other state.

"Since 1970 there's been a sunstantial reorientation of black migration flows," said Larry Long, chief of the population analysis staff at the Census Bureau. "It used to be that there were relatively few new employment opportunities for blacks in southern states like Virginia. Now jobs are increasing more in the South than elsewhere in the country, and more jobs are open to blacks (because of desegregation)."

Long added that the net black inmigration to Virginia since 1970 also reflects the movement of blacks from Washington to the Northern Virginia suburbs. The same sort of suburban movement, on an even larger scale, is going to the District's Maryland suburbs, he noted, explaining most of the heavy black inmigration to that state.

"The blacks of Washington are now following the same patterns that the whites took in the 1950s and 1960s," Long said. "More and more blacks are in middle-income levels and can afford suburban housing, and the housing is more available to them."

The new estimates are based on complicated formulas using birth and death statistics, and data on federal income tax returns, social security recipients, and inmigration.

Washington is the only city or county for which the census bureau had made racial estimates because, for the purpose of population reports, the District is considered to have the same status as the 50 states.

However, according to estimates released by the state of Virginia, the black population of Northern Virginia increased by about one-third from about 60 percent in Fairfax county.

William J. Serow, director of the Tayloe Murphy Institute at the University of Virginia, which prepares the state estimates, said the Richmond, Norfolk, and Newport News metropolitan areas also attracted substantial numbers of blacks from 1970 to 1975.

Many of them, he said, come from heavily black rural areas of the state, where most of those who left traditionally went to industrial cities of the North and Midwest.

"The migration of blacks out of these rural areas is still going on," Serow said, "but the rate is much less than it used to be, and the people are going to our own urban areas rather than the North."

Huey J. Battle, director of the Bureau Economic Research and Development at Virginia State College, said the main reasons for the end of the black migration flow out of Virginia, which had persisted since the Civil War, were the state's thriving economy and the end of legal segregation.

"The economic development is here now," Battle said, "and Virginia now is a more condicive place for blacks to live than it was in the past. There's still a great deal to be done, but it's much better."

According to the new census data, white migration into Virginia continued at about the same rate from 1970 to 1975 as it had been during the 1960s. But overall the state's black population increased faster than its whites - 8 percent compared to 6.4 percent - for the first time this century.

In Maryland, the census bureau reported, 38,700 more whites left the state than moved in from 1970 to 1975, a major change from the net immigration of 290,000 whites during the 1960s.

Although racial estimates for individual counties are not available from the census bureau, figures released by the Maryland State Health Department indicate that Prince George's was the county where most of the turnaround took place. Prince George's also was the chief destination of the large black movement into the state.

According to the state government estimates, the number of whites living in Prince George's fell by 73,700 or 13.1 percent from 1970 to 1975 after increasing by 236,727 during the 1960s. The county's black population rose by about 91,000 during the first half of 1970s, compared to an increase of 60,797 during the previous 10 years.

David Word, a census bureau demographer who was in charge of preparing the state-by-state racial estimates, said the steep growth in the black population of Prince George's is "a natural spillover across Southern and Eastern avenues" from the District.

"It shouldn't surprise anyone," he said. "I think it's going to continue."

The census bureau's Long said "the most plausible hypothesis" for the drop in Prince George's white population is that the county "is looking more and more like a central city to them and they are leaving for the same reasons as people left central cities . . . People feel the schools are deteriorating somewhat and that crime is rising. When they can afford to, they're moving away."

According to the state data, Montgemery County also has had a major increase in blacks - about 25,000 from 1970 to 1975. But its white population continued to increase though much more slowly than in the 1960s.

In Washington the net outmigration of blacks during the first half of the 1970s amounted to 9.9 percent, the bureau reported, compared to a net inflow of 8.7 percent during the 1960s.

On the other hand, the net outmigration of whites from 1970 to 1975 (14,400) was 6.8 percent - just about one-tjird of the rate of their exodus during the 1960s.

In addition, the census bureau said that 7,100 more white residents died in the city from 1970 to 1975 than were born there, producing an overall drop in the white population of 21,500 or 10.2 percent.

The city's black had 26,800 more births than deaths during the same five years but because of outmigration the black population fell by 26,500 or 4.9 percent.

Several months ago, the District of Columbia's government estimated that the number of whites in Washington rose slightly in 1976 for the first time in a quarter century.

However, even with the increase the D.C. government figures, which are based on a different methodology than the census bureau used, show 20,000 fewer whites than the 189,800 in the new census estimates.

By both estimares, whites account for about a quarter of the city's residents.

States that in the new census report showed a net outflow of blacks during the first half of the 1970s in contrast to earlier growth are Connecticut, Illinois, Kansas, Missouri, Ohio, pennsylvania, Rhode Island and Wisconsin.

Because of heavy immigration, particularly from Vietnam, the new census estimates show a 35 percent rise in the Asian and "other race" population throughout the United States from 1970 to 1975.

According to the report, Virginia, Maryland and the District shared in the increase although Asians still account for only about 1 percent of their population.

The "other race" group includes American Indians. Hispanics are included among either whites or blacks and are not listed separately.